The present invention relates to the display of messages on a display device of the scanned raster type such as a cathode ray tube (CRT) used, for example, as a terminal in a data processing system. More particularly, the invention relates to the editing, the transmission and the reception of messages containing page composition data to be displayed on the screen of a CRT. A system for the embodiment of these operations is commonly called a "teletext" system.
There are, generally, two modes for displaying messages on a CRT, the alphanumeric mode and the graphic mode. Referring to FIG. 1 of the drawings, messages in the alphanumeric mode comprise a plurality of rows 1 of characters, each character being defined by a matrix 2 of N lines and M dots. The matrices are stored in one or more character generators which are formed by read-only memories or by corresponding zones of a composite memory associated with a CRT display controller. The character generators have addressable positions in which the data necessary to display each alphanumeric character are stored.
To display a page of text, the system includes at least one page memory which is a read/write memory (or RAM) for storing the codes of the alphanumeric characters to be displayed. Each character thus coded is formed of two parts, one of which is the character code, for example in ASCII code, and the other is an attribute code relating to the color, the background and other functions to be embodied when the character is displayed. FIG. 2 shows the configuration of the coded characters stored in a page memory. Each coded character 3 corresponds to an alphanumerical character in a row and is formed of an attribute code 4 and a character code 5. The coded characters are arranged in the memory in locations corresponding to those of the characters displayed on the screen.
Consequently, whenever a page composed of X rows of Y characters is to be displayed, the page memory contains XY coded characters. The successive addressing of these coded characters permits access to the character generator thereby producing the display. As this operation progresses, the image is built up on the screen by scanning of a raster composed of intersecting lines, as is well known in video technology. Since the matrices are composed of N lines, each row of characters is thus displayed by N successive addresses of all the matrices of characters in a row, this addressing being carried out at the line scanning rate of the CRT. After the display of a complete row, the following row is displayed line by line, like the preceding row.
In the graphics mode the character generator is not used. Instead, a page memory containing the page data in the form of separate dots 6 (FIG. 3) coded in binary fashion is employed. This code is a function code relating to the color or the shade of gray of the dot to be displayed. This method permits any form to be displayed on the screen within the limits of resolution dictated by the density of the dots on the screen. The dots are stored in locations corresponding to their position in the displayed image. The page memory is read line by line at the CRT line scanning rate.
By way of example, referring to FIGS. 1 and 2, each matrix 2 of characters stored in the character generator and is played on the screen is formed of 8.times.10 image dots (M=8, N=10), and each row contains 40 characters (Y=40). Moreover, each character 3 requires a character byte 5 and an attribute byte 4 for its identification in the page memory. Furthermore, each page contains 25 rows of alphanumeric characters (X=25). Under these conditions, in the alphanumeric mode, the page memory must have a capacity of 25.times.40=2000 bytes for the display of a complete page.
In the graphic mode a three-bit code may be used for each dot to provide for eight shades of gray or eight colors of a number of dots equal to that necessary for writing 40 matrices side by side. In this example, each line requires 960 bits, or 120 eight-bit bytes per line. For an equal number of image lines (25.times.10), the page memory then stores 30,000 bytes, assuming the entire page contains graphic elements.
Because of the difference in the implementation of the display in the alphanumeric and graphic modes, it is not possible in prior systems to mix on a given page alphanumeric data and drawings, graphs or other forms composed of lines, because all of the data relative to the display of a given page must be stored beforehand in a page memory, so that it must be determined in advance whether the page is of an alphanumeric or a graphic nature. Thus, in order to add graphic drawings to a message of text, it is necessary to use separate pages for the alphanumeric portion and the graphic portion which requires a considerable increase in the capacity of the page memory. Further, in order to obtain an appropriate display, it may be required to load two page memories alternately, which would necessitate a memory with a minimum of 60K bytes in order to display in both the graphic mode and the alphanumeric mode on the same screen.
Another problem involved in graphic mode display is the relatively long time required for the receiving system to store the data in its page memory for display. Of course, the graphic mode also necessitates constraints at the sending level, because for a page in graphic mode the time required in the sending system is much longer than that required for sending a page in the alphanumeric mode. The graphic mode, therefore, considerably reduces the density of the data that can be transmitted per unit of time.
Yet another problem related to the implementation of teletext systems, and to data processing systems in general, is the exchange of data between two systems having different data processing rates, for example, a central unit and a terminal. Prior art systems have employed a plurality of registers used as a buffer between the two processing systems, the buffer operating in a first-in-first-out (FIFO) manner. However, this method is relatively high in cost, low in capacity and cannot be used unless the data processing rates do not differ too widely.